The invention relates to reusable heat packs and, more particularly, to heat packs containing supercooled solutions which liberate heat when caused to crystallize.
Heat packs of various types have long been used in the medical field and by sportsmen for the purpose of applying heat to localized areas of the body to, for example, alleviate stiffness and minimize damage due to freezing of the skin. The simplest form of a heat pack is a hot water bottle. Although water has a high heat content, the temperature of the water is difficult to control for any length of time and heating of the water may be impossible when most needed, as when the need arises to treat frozen appendages in the field.
To provide improved heat sources, heat packs have been developed using aqueous salt solutions which can be supercooled so that the heat packs can be carried in the supercooled condition and activated with internal release of heat when desired. Sodium acetate and calcium nitrate tetrahydrate are examples of suitable salts.
With respect to the use of supercooled sodium acetate solutions in heat packs, activation or crystallization initiation has presented a problem. Various techniques have been recommended including: contacting some of the supercooled solution with air; inserting a crystal of material into the supercooled solution; and scraping the inside of a container made from a hard material such as metal with a wire to introduced impurities into the supercooled solution. Examples of these techniques are disclosed in the following U.S. Pat. Nos. 1,433,010; 1,915,523; 2,289,425; 2,220,777; and 3,093,308. The existence of many such patents over a long time period is indicative of the continuing need for a simple, effective means for activating supercooled aqueous sodium acetate solutions so that more versatile heat packs can be developed using such solutions.
Aforementioned U.S. Pat. No. 2,220,777 discloses a self-contained heater which, in one embodiment shown in FIG. 12 thereof, may be flexible, but which requires at least two metallic components which must be scraped together by a user in order to activate a supercooled solution contained therein. The need to use at least two metallic components and the necessity of scraping them together can place undesired restrictions on the design of the heater. For example, heat loss will be greater as the number of metallic components increases or it will be more expensive to insulate the heater against such heat loss. Additionally, heater size and/or its fluid content will be limited because of the need to be able to bring the two metallic components together.